Transformations of heavy metals added to soil: Application of a new sequential extraction procedure
Ma, Y.B.; Uren, N.C.
Geoderma 84(1-3): 157-168
ISSN/ISBN: 0016-7061 DOI: 10.1016/s0016-7061(97)00126-2
A sequential extraction procedure, used to remove the heavy metals in specifically adsorbed and easily reducible manganese (Mn) oxide fractions, was used to study the transformation of heavy metals added to an alkaline soil. Most of the endogenous Cu (86%) and Pb (79%) were found in the residual fraction (RES) which was considered to be mainly highly crystalline Fe oxides and silicate minerals. The recently added Cu, Pb and Cd existed mainly on the surfaces of the soil particles as reactive fractions (water-soluble plus exchangeable and NaCaHEDTA-extractable fractions) and as highly stable forms (RES fraction). There was a particularly high affinity of Pb for Mn oxides. The concentrations of metals in the reactive fractions were in the order: Cd > Cu > Pb. When water-soluble heavy metals are added to the soil, they are rapidly retained by the soil. The reactive forms then slowly transform into highly stable forms. The processes associated with the transformation of added Cu and Pb can be described by a diffusion equation. The processes may be attributed mainly to the diffusion of the surface species into micropores and the entrapment in microporous solids. Unlike Cu and Pb, most of the exchangeable Cd transformed to the forms extracted with NaCaHEDTA and to residual forms. The slow processes of the transformation of Cd added to soil may be attributed to inner-sphere surface complexation via partial or complete dehydration of surface species. The relative diffusion rate coefficients (D/r2) were found to be of the order of 10-10 to 10-11 s-1. Addition of CaCO3 decreased the reactivity or extractability of added heavy metals through the increase in pH.